Process and apparatus for the continuous production of aqueous solutions of sodium hydrosulfite from sodium amalgam and gaseous so2



1956 A. BATTARRA ETAL 2,773,740

PROCESS AND APPARATUS FOR THE CONTINUOUS PRODUCTION OF AQUEOUS soumous 0F SODIUM HYDROSULFITE FROM SODIUM AMALGAM AND GASEOUS so;

Filed March 19, 1953 PROCESS AND APPARATUS FUR THE CONTINU- OUS PRGDUQTION F AQUEGUS SOLUTIONS "0F SGDIUIVI HYDROSULFITE FROM SODIUM AMALGAM AND GASEOUS S02 Arnoldo Battarra, deceased, late of Milan, Italy, by Marie Elisa Battarra, heir and guardian of heirs, Alberto Battarra, heir, Dino Maveri, Alfonso Melloni, and Luigi Frigeri, all of Milan, Italy, assignors to Montecatini, Societa Generale per llndustria Mineraria e Chimica,

. a corporation of Italy Application March 19, 1953, Serial No. 343,334 Claims priority, application Italy March 21, 1952 7 Claims. (21. 23-116) The present invention relates to the production of sodium hydrosulfite in aqueous solution, from sodium amalgam and gaseous S02 in the presence of an aqueous solution of sodium bisulfite and sulfite or buffer solution according to the following equations:

The direct reaction between sodium and S02 offers no industrial interest because operation has to be carried out under conditions, which yield a hydrosulfite that is too much polluted with products of decomposition so that it cannot be utilized in practice; but also the use of an aqueous solution of sulfite and bisulfite acting as a vehicle-owing to the intervention of numerous parasitic secondary reactions (about twenty are possible) complieating the probleminvolves difiiculties which have so tarhindered the industrial utilization of that play of reactions so simple in appearance and verified in the laboratory.

The technological requirements (due also to the properties'of sodium hydrosulfite) which as is known must all bernet in order that a process based on the reactions Written above may be practicable, are the following:

a (:1) The time of stay of the hydrosulfite in the reactor has who as short as possible, to prevent its decomposition (formation of thiosulfates, etc.); (2) The hydrosulfite solution obtained has to be as concentrated as possible in order that the so called saltingout may be feasible, that is the separation by means of an addition of sodium chloride, which is successful with industrially acceptable yields only under determined conditions of concentration;

(3 The hydrosulfite obtained has to be as pure as possible in particular the less possible polluted with sulfite,

because subsequent separation of the latter is impossible inpractice.

From both the point of View of apparatus equipment (there are not known any proposals other than the generic proposal of stirring in a cooled reactor containing the vehicle-buffer solution over the amalgam fed and taken Ofi lirom bottom, while the gaseous S02 is fed from top) and the point of view of process (the most recent patents propose for example the addition of other neutral and slightly acid alkali salts to the solution, temperatures of from to 14 0, pH from 6 to 7); it can be said that the search for operating conditions such as to attain high yields and, at the same time, high concentrations, has

- not succeeded up till now in providing a result translatable into industrial scale.

Now it is an object of the present invention to provide a combination of process and apparatus characteristics United States Patent 2,773,740 Patented Dec. 11, 1956 "ice representing an improvement of operating conditions enabling to utilize eventually the laboratory reactions on industrial scale.

it has been found in fact that one of the essential conditions for eliminating the inconveniences encountered heretofore, is an intensive particular circulation of the bufier solution so that it brings the latter into rapid alternating contact with the reagents, S02 and sodium amalgam.

So far, the importance of circulating. the solution between the surfaces of contact between S02 and the amalgam respectively had not been recognized. In the known proposals, the efiect is not even considered; and other features of process and apparatus of the present invention are lacking therein as well.

Still according to the present invention, the process may be carried out with two reactors in which separately the solution reacts with S02 and with sodium amalgam respectively, as well as with one single apparatus having suitable characteristics. In the case of the variant with two distinct reactors, an intensive circulation has to be ensured also therebetween.

Still according to the present invention the other conditions under which these reactions have to be carried out to attain the desired result, are:

(1) Temperature from 10 to 25 0, preferably higher than 15 C.;

(2) pH kept in the limits between 5.4 and 5.8;

(3) Contact between the solution and S02 on a surface of frustum of paraboloid of rotation in rapid movement and having such surface area that its ratio to the surface area of the liquid if at rest is at least 1.2: l;

(4) Exclusion of the central zone of the paraboloid, corresponding to its vertex, obtained by means of a convenient projection on the bottom of the reactor;

(5) Sodium amalgam:

(a) In thin film (from about 2 to 5 mm.);

(b) Circulating by gravity in an inclined peripheral channel;

(0) Taken off not completely exhausted (sodium concentration about 0.001 to 0.002% by weight against about 0.01 to 0.05 at the inlet);

(d) Not stirred.

The taking oil of the final solution is effected preferably from the top of the reactor containing the amalgam, or of the single reactor respectively.

As for the concentration of the buffer-vehicle solution, it may be selected in accordance with the type of product solution that has to be obtained (concentrated for saltingout or diluted for particular uses) the difficulties encountered heretofore on industrial scale being avoided with the process of the invention. So, for example, if operation is carried out under the conditions specified here inbefore, starting from bufier-solutions of sodium sulfite and bisulfite with SOz-concentrations up to 70 g./l. and introducing S02 in stoichiometric proportion in respect to the sodium fed in the form of an amalgam, it is possible to obtain hydrosulfite solutions at the limit of solo bility, capable of giving a finished solid product with an NazSzOr content higher than 85%, with overall yields between 50 and Analogously starting from bufier solutions containing at least 15 g./l. of S02 and still under the specified condi tions, it is possible to obtain with yields of -80% hydrosulfite solutions with at least 40 g./l. suitable as they are for bleaching.

The invention with these and other features will be more clearly explained hereinafter with reference to the accompanying drawings wherein:

Fig. la is a schematic view of one embodiment, show? amalgam'takes place. Within the apparatus there is the stirrer 2 which renewing the solution in the contact zone accelerates the reaction between the amalgam and the solution. The amalgamated mercury flows by gravity within the apparatus through a circular channel 3, into which it is fed through the inlet 4 and which it leaves through the outlet 5, while a bafiie 6, see Fig. 1b, has the task of preventing the amalgam from passing directly from the feed 4 to the discharge 5 without flowing through the channel 3. The solution contained in the apparatus 1 is sucked through the piping 7 by the pump 8 and is sent through the piping 9 to the cooling coil (not shown in the drawing).

The cooled solution then enters the apparatus 10 equipped with a powerful stirrer 11, which imparts to the surface of contact between the liquid and gaseous phases the shape of a frustum of rotation paraboloid.

It is in this apparatus 10 that the solution comes to contact the S02 in gaseous phase, introduced by way of the piping 12.

The solution that has absorbed the S02 leaves the apparatus 10 through a syphon pipe 13 operated by overflow (with a connecting piping 14 to avoid priming of the syphon). The solution returns by gravity to the apparatus 1. Also in the apparatus 1 it is advantageous to have such agitation as to give the liquid at surface of frustum of paraboloid.

The feed, constituted by a solution of sodium sulfite and bisulfite, is introduced into the apparatus 10 from 15, with a hydraulic valve (not shown).

The taking off of the product solution may be obtained in a continuous manner from the apparatus 1 through the overflow piping 16. The apparatuses 1 and 10 are run in continuous cycle.

In Figures 2a and 2b there is represented an improved embodiment of apparatus permit-ting to practice the conditions of circulation according to the invention in one single reactor.

With reference to Figures 2a and 2b, the apparatus 17 is constituted by a cylindrical vessel having a fiat bottom and closed at top by a cover 18 through which passes the shaft of the stirrer 19 and on which there are applied the inlet 26 for feeding S02 in the gaseous state, and the outlet 21 venting to the atmosphere through an ordinary hydraulic valve not represented in the drawing; on the bottom there is applied a profile 22 with a circular step and a peak at the centre. On the circular step channel flows the amalgam; this step is inclined in such a manner that the amalgam acts as a thin film of from2 to5 In fact, the amalgam enters from the inlet 23 into the inclinedchannel 24 and passes to the outlet 25, which is separated from the inlet 23 by means of a bafl le 26.

The profile 22 has a peak at its centre which rises above the channel by an amount ensuring that during operation, viz. during formation of the already described frusto-paraboloid surface, there remains no liquid at the centre where stirring is less efiective. T

The stirrer 19 is constituted by blades shaped in such 7 surface between said bufier solution ands-aid gaseous S02 constitutes a frustum of a rotation paraboloid that the vsur- 27 placed outside the apparatus, into which the cooling fluid enters at 28 and which it leaves at 29. The ex-.

changer surface is dimensioned in such a Way as to keep the temperature of the liquid in the reactor between about 10 and 25 C., preferably between 15 and 25 C.

The circulation of the solution in the cooler 'may be effected with a propeller or even without, merely-utiliz ing the rotation of the very solution within, the reactor,

This is possible by arrangingas caused by the stirrer. I the outlet piping 30 and the return piping 31 tangential to the apparatus and as shown in the, drawing. The ap paratus is fed in a continuous manner with buffer solution by means of an inlet 15, while the hydrosulfite solution is removed continuously through an outlet 16. i

The corresponding S02 is fed in the gaseous phase through the inlet 20. Also this apparatusrhas the char The dis charge of product is effected continuously througha conventional overflow and syphon not represented in the acteristic of operation in continuous cycle.

drawing.

In this apparatus an intensive circulation of the solution between the surfaces separating the various phases is obtained. As for the potentiality of the reactor, it is in relationship with the diameter'of the latter; with adiameter of 500 mm., the apparatus can treat with satisfactory yields 2 to 3 kg./h.; with a diameter of 700 mm., up to 3-4 kg./h.; with a diameter of 1000' mm. up to 5-6 kg./h. of sodium. 7

These reactors are suitable for -preparing hydrosultite solutions having suflicient concentration (-160 g./l.

of NazS2O4) to permit preparation of solid sodium hydrosulfite (salting-out), as well as for preparing more diluted solutions (40-100 g./l. of Na2SzO4) fordirect use in paper-mill bleaching or other analogous applications, as already mentioned above.

It is important that the amalgam should be discharged before being completely exhausted.

Also in the case of Figures 1a and 1b, the

profiled like those of the reactor of Figures 2a and 2b. V In conclusion, the contrivances constituting thecharacteristics of the present invention provide'operatin'g' conditions for the formation of hydrosulfite such as to balance the partly contrasting efiects of the essential factors influencing this 'productiom-maximum reduction ofimpurity contents in the product; minimum decomposition of the hydrosulfite in the reactor; elevated concentrations.

of the product and, at the same time, elevated yields; so as to permit practicing the process on industrial scale. 7

We claim: 1. The process of producing an aqueous solution of loid. v

2. The process of producing an aqueous solution of sodium hydrosulfite by reacting in an enclosedsy'stem an aqueous butter solution of sodium sulfite and sodium bisulfite with sodium amalgam and gaseous S02, said process comprising rapidly circulating said bufier solution between contact surfaces of said sodium amalgam and.

said gaseous S02, whereby the contact surface between said'bufier solution and said sodium amalgam constitutes an undisturbed circular peripheral ring' andrthe contact has a surface area at least 1.2 times as'large as face area of the buffer solution at rest. I

reactors (1 and 10) may have conveniently bottoms and stirrers 3. The process of producing an aqueous solution of sodium hydrosulfite by reacting an aqueous buffer solution of sodium sulfite and sodium bisulfite with sodium amalgam in one part of an enclosed system, transferring said solution to another part of the enclosed system and reacting it there with gaseous S02, said process comprising rapidly circulating said buffer solution between contact surfaces of said sodium amalgam and said gaseous S02, whereby the contact surface between said buffer solution and said sodium amalgam constitutes an undisturbed circular peripheral ring and the contact surface between said bufier solution and said gaseous S02 constitutes a frustum of a rotation paraboloid.

4. In a process which comprises reacting an aqueous buffer solution of sodium sulfite and bisulfite with sodium amalgam and with gaseous $02 on distinct and separate contact surfaces, the step of circulating sodium amalgam in an undisturbed thin annular film of from about 2 to about 5 mm. thickness in contact with sodium sulfite and bisulfite solution, said amalgam being continuously fed with a sodium concentration of from about 0.01 to 0.05% by weight and discharged before being completely exhausted, with a sodium concentration of from about 0.001 to about 0.002% by weight.

5. The process of producing an aqueous solution of sodium hydrosulfite which comprises reacting an aqueous buffer solution of sodium sulfite and bisulfite with sodium amalgam and gaseous S02 on distinct and separate contact surfaces in rapid relative movement, the contact surface between said solution and said sodium amalgam being that of a circular peripheral ring and the contact surface between said solution and said gaseous 502 being that of a frustum of a rotation paraboloid, whereby said solution is intensively circulated between said two contact surfaces, the pH of the medium is maintained between about 5.4 and 5.8 and the temperature between about and 25 0., preferably above C., and the gaseous S02 being fed in substantially stoichiometric proportion in respect to sodium.

6. A reactor for producing sodium hydrosulfite by reacting a fluid constituent consisting of a buffer solution of sodium sulfite and bisulfite with gaseous S02 and with sodium amalgam on distinct contact surfaces while rapidly circulating said fluid constituent between said surfaces, said reactor comprising a cylindrical, enclosed vessel provided with a raised, central bottom portion of substantially conical shape, surrounded by a peripheral annular channel, inlet and outlet openings at the bottom of said channel adjacent to each other and separated by a baffle within the channel, a stirrer comprising a shaft and stirrer arms sloped to rotate parallel to, and in close proximity with, the surface of said raised central bottom portion and extending horizontally over said peripheral annular channel at a height to clear said baflle, an inlet at the top for gaseous S02, a vent at the top, ex ternal cooling means for cooling the zone of the reactor containing said fluid constituent, and an outlet in the reactor wall for removing the fluid constituent.

7. A reactor for producing sodium hydrosulfite by reacting a fluid constituent consisting of a buffer solution of sodium sulfite and bisulfite with gaseous S02 and with sodium amalgam on distinct contact surfaces while rapidly circulating said fluid constituent between said surfaces, said reactor comprising a cylindrical, enclosed vessel provided with a raised central bottom portion of substantially conical shape, surrounded by a peripheral annular channel, inlet and outlet openings at the bottom of said channel adjacent to each other and separated by a baflie within the channel, a stirrer comprising a shaft and stirrer arms sloped to rotate parallel to, and in close proximity with, the surface of said raised central bottom portion and extending horizontally over said peripheral annular channel at a height to clear said baflle, an inlet at the top for gaseous S02, a vent at the top, external cooling means for cooling the zone of the reactor containing said fluid constituent, said cooling means comprising a separate heat exchanger and conduits arranged between said reactor and said heat exchanger to form a loop tangential to said reactor for circulating a portion of said fluid constituent between said reactor and said heat exchanger, and an outlet in the reactor wall for removing the fluid constituent.

References Cited in the file of this patent UNITED STATES PATENTS 1,477,130 Kuhne Dec. 11, 1923 1,609,773 Riggs Dec. 7, 1926 2,084,651 Mecklenburg June 22, 1937 2,204,476 Dorph June 11, 1940 2,576,769 Avedekian Nov. 27, 1951 

5. THE PROCESS OF PRODUCING AN AQUEOUS SOLUTION OF SODIUM HYDROSULFITE WHICH COMPRISES REACTING AN AQUEOUS BUFFER SOLUTION OF SODIUM SULFITE AND BISULFITE WITH SODIUM AMALGAM AND GASEOUS SO2 ON DISTINCT AND SEPARATE CONTACT SURFACES IN RAPID RELATIVE MOVEMENT, THE CONTACT SURFACE BETWEEN SAID SOLUTION AND SAID SODIUM AMALGAM BEING THAT OF A CIRCULAR PERIPHERAL RING AND THE CONTACT SURFACE BETWEEN SAID SOLUTION AND SAID GASEOUS SO2 BEING THAT OF A FRUSTUM OF A ROTATION PARABOLOID, 